A variety of different applications use sensor systems to detect the movement of an underlying object or the presence of a substance or condition in a particular environment, such as sensors that detect motion, light, pressure, humidity, sound and gases. Sensors employing microelectromechanical systems (MEMS) devices are increasingly used in such applications due to their relatively small size and their capability to detect relatively small amounts or changes in the measured item.
MEMS devices typically employ a movable, inertial mass or flexible membrane formed with one or more fixed, non-moving structures or fingers. For example, in a MEMS accelerometer, the inertial mass may be suspended in a plane above a substrate and movable with respect to the substrate. The movable structure and the fixed structures form a capacitor having a capacitance that changes when the movable structure moves relative to the fixed structures in response to applied forces, such as along a predetermined axis of the device, e.g., x- and y-axes. For example, commercial MEMS accelerometers that sense linear motion in the x-, y- and z-axes may have electrodes positioned above, below and/or on opposing sides of the inertial mass to allow measurement of differential capacitance in each axis. This type of design generally offers high sensitivity to linear acceleration with minimal temperature or stress sensitivity. Because of the mechanical moving structures involved and the typical required device sensitivities, MEMS devices are commonly covered with a cap structure to protect the MEMS structures from hazards that may impact the functioning of the device, e.g., from gases, particles, moisture, etc.
MEMS devices are commonly made by a sequence of thin film depositions and etches performed on a substrate. Typically, the substrate is formed from a single crystal silicon wafer or a silicon-on-insulator (“SOI”) wafer. As known by those in the art, an SOI wafer has an insulator layer between two single crystal silicon layers with the inertial mass and fixed fingers typically formed in the top silicon layer. Designs based on SOI wafers may measure in-plane (x- and y-axes) acceleration, but are generally less satisfactory for measuring out-of-plane or z-axis movement. This is because it is difficult to form an electrode under the inertial mass. Solutions such as the widely known “teeter-totter” do exist. One can also use the handle/bottom silicon layer as the ground and drive the inertial mass to sense z-axis motion capacitively. This approach, however, is susceptible to various drift mechanisms.